As is well known in the art, an automatic transmission realizes automatic shifting to an appropriate speed according to driving states of a vehicle. For such automatic shifting, an automatic transmission includes at least one friction element therein, and also a hydraulic pressure circuit for hydraulically controlling the friction element.
In order to control hydraulic pressure supplied to the friction element, such a hydraulic pressure circuit includes at least one solenoid valve therein, and an operation of the solenoid valve is controlled by a separate electronic control unit (usually called a transmission control unit). Typically, the transmission control unit controls the solenoid valve by controlling a current supplied to the solenoid valve. In response to the control of the current, the solenoid valve controls hydraulic pressure input thereto.
The hydraulic pressure-current characteristic of the solenoid valves, i.e., the relationship between the current applied to the solenoid valve and hydraulic pressure output from the solenoid valve according to the applied current, always lies within some tolerance range even though the solenoid valves are designed to the same specification. Therefore, if a solenoid valve actually used in the automatic transmission shows a different hydraulic pressure characteristic from a designed specification, the automatic transmission does not provide precise shift quality as designed.
If automatic transmissions are controlled with the same preciseness and stability against the tolerance of solenoid valves, it means an enhancement in the quality of the automatic transmissions. Furthermore, it also means that cheaper and simpler solenoid valves having larger tolerance may be used in an automatic transmission without deteriorating shift quality. Accordingly, it enables a reduction in production cost of an automatic transmission.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.